I am not an SEO specialist. I am not even an SEO sympathiser. In fact I often use my website to debunk "facts" that I am told regarding SEO. So don't take this as SEO advice. It is just an observation.
Recently, I got a bit fed up with having to log into a CMS to edit pages. What I really wanted to do was edit source directly, and I set about making this possible, then the penny dropped and I realised what I actually wanted was a non CMS website, with no router, no databases and no rich text editor. I like editing files directly. It works and I don't have to figure out what to do to get the CMS to generate the HTML I could write by hand.
So two things happened on the same day, as far as search engines were concerned. All my URLs changed with no 301 redirects in place and secondly all my pages suddenly got a lot smaller and tidier, although the content remained exactly the same.
A few days later this is what happened to my visits
Now you'd expect with me breaking all my URLs like that, Google would get upset. What no 301 redirects? All those 404 pages? Oh my goodness.
So did the clean up of the source cause the change? I'm not sure but it looks like it did. I have a few other websites I can play with that are currently built on CMS so I will port those to simple files and see if I get similar results.
UPDATE Jan 2015: Here is a follow up post discussing another site I did this on, and the effect it had.